The circuit terminals of a printed circuit (PC) module or board usually take the form of an array of conductive pads positioned on the module, often at an edge thereof, so that the module circuit can be connected to external circuitry by coupling the module to or plugging it into an electrical connector. The connector is usually mounted to the end of a flexible cable or harness and it has an array of internal spring-like contacts which are connected to the various cable conductors. When the connector is properly coupled to the printed circuit module, its contacts resiliently engage the terminals or pads of the module. Usually each contact is formed with a raised boss or bump which is the actual point of contact with the corresponding pad of the module.
Under optimum conditions, when the connector and module are properly aligned, all of these contact points are more or less centered on their corresponding module pads so that reliable, low-resistance electrical connections are made between the connector contacts and those pads. However, if the module is coupled to or plugged into its connector improperly or incompletely, one or more of the contact points may overhang the edge of its module pad raising the contact resistance of that connection or the contact point may miss the pad entirely creating an open circuit. Connector-module misalignments have historically been a major source of system failure particularly in the computer industry and these failures routinely occur both before system power up and after same.
Until now, before power up, the principal way to verify that all of the proper electrical contacts between a connector and the pads of a PC module have been made has been to couple the connector and module together and then separate them and check the scratches or "witness marks" made on the module pads by the connector contacts. This verification technique has an inherent disadvantage in that it requires the module to be removed from the connector in order to observe that all of the contact points were aligned with the module pads. Obviously, misalignments may occur if the module is plugged back into the connector improperly. Also, even after the system is powered up, a failure can occur due to misalignment of a connector and its module caused by vibration of the equipment or thermal changes, for example.
There do exist in the art of various instruments for testing electrocontinuity between printed circuit modules and their connectors. See U.S. Pat. No. 3,784,910, for example. However, these tend to be complex and costly pieces of equipment and the testing procedure is quite time-consuming due to the large number of such modules present in a typical computer or electrical system.
U.S. Pat. No. 4,507,697 discloses a device for verifying continuity between a circuit board or card and its connector indirectly by indicating the position of the circuit card with respect to its connector. That particular device is able to detect three positions of the card relative to its connector. In the first or correctly installed position, good safe contacts between the corresponding conductor elements of the connector and card are assured; in the second or intermediate position, the contacts between the corresponding conductor elements of the connector and card are not safe, as when a card is being inserted or removed with respect to its connector, so that the connections cannot be viewed as assured, and in the third position, a card is completely removed from its connector so that no contact is established between the corresponding conductor elements of the connector and the card. By accurately knowing the position of each card, power can be turned on to each card position selectively without shutting the entire system down.
That prior apparatus detects the position of the card by means of a switch actuator that engages a notched edge of the card which functions as a cam surface that defines the three positions of the card relative to the connector. Thus, that arrangement requires a mechanical switch actuator at each card position, which actuators take up needed space. Also, while that apparatus can detect whether or not a card is plugged into its connector, it cannot necessarily verify that a particular card is plugged in completely so that all of the contacts of the connector are disposed opposite the corresponding terminal pads of the card or module.
The above described prior testers and position indicating devices are particulrly disadvantaged in that they do not allow for the monitoring or tracking of small shifts in the position of a card or module that occur after system power up and during normal operation of the system. Accordingly, they cannot be used to detect incipient connector-module misalignments so that prompt corrective action can be taken before a failure actually occurs.